Let me just say that I'm currently transitioning back to my trainer but as weather and time allows I'm riding outside. I don't want to be bothered fooling with the trainer anymore than I have to, so a dedicated bike is very useful to me. I would say that by November 15 I will be basically riding inside all the time with some but relatively few outside rides for about 5 months. I'm really a big advocate of trainers, it has improved my riding so much. Having something like Zwift to go along with the trainer makes it tolerable and at times even fun.
It's not the same as outside but last year being my first winter on a trainer I went into the winter as a "meh" rider, exited the winter a reasonable decent recreational rider and vastly improved my ability to pull the hills. I'm currently in the process of putting together a training plan for this winter. Since I have all of the Zwift bonus XPs from riding routes the first time and have all of the challenges completed (California, Italy, Everest/Tron) I'm going to have to make my own goals as moving up levels is really difficult without bonus XPs. I'm going to ride up the Alpe once a week and Ven-Top twice a month, will probably do intervals and crank up short hills like Box Hill with all I've got. I really want to get my outside flat terrain fun ride of 40-60 miles up to as close to 20 MPH average as I possibly can for next year. I can't do that without training indoors over the winter.